1. Field of the Invention
The present invention relates to a distortion compensating apparatus and method for compensating distortions of an amplifier that amplifies analog signals converted from digital input signals.
2. Description of the Related Arts
It is ideally desirable that amplifiers for amplification and output of input signals have a proportional relationship (linear relationship) between its input signal power (input power) and output signal power (output power) as indicated by an imaginary line (chain double-dashed line) in FIG. 15.
The actual amplifiers however generically have input/output characteristics indicated by a solid line in FIG. 15, i.e., in the relatively small input power region (linear region) the input power is proportional to the output power whereas in the relatively large input power region (nonlinear region or saturated region) the input power is not proportional to the output power but the output power tends to become saturated with larger distortions accordingly as the input power increases.
In order to use the amplifiers with a high energy efficiency, use in the larger input power region is preferred and hence the amplifiers are typically used in the nonlinear region. Transmitters are thus provided with a distortion compensating apparatus arranged to compensate output signal distortions in the nonlinear region so as to allow the output signals to be proportional to the input signals.
FIG. 16 is a block diagram showing the configuration of a transmitter equipped with a conventional adaptive predistortor type distortion compensator which is one of the distortion compensating apparatuses.
This transmitter has input terminals Si and Sq that receive an I channel (Ich) digital signal xi and a Q channel (Qch) digital signal xq, respectively, both in the form of base band signals. These signals are fed to a power calculating unit 100 and a multiplier 102.
The power calculation unit 100 finds power values p of the fed digital signals xi and xq and feeds the power values p to a distortion compensation coefficient storage unit 101. The distortion compensation coefficient storage unit 101 has distortion compensation coefficients which correspond to various input power values p and, when a power value p is received, it feeds to the multiplier 102 a distortion compensation coefficient h (hi corresponding to Ich and hq corresponding to Qch) corresponding to that power value p.
The multiplier 102 multiplies the digital signals xi and xq with the distortion compensation coefficient h and feeds the results of multiplication to digital-to-analog converters (hereinafter referred to as DACs) 103i and 103q, respectively. The DACs 103i and 103q convert input digital signals into analog signals and feed the analog signals to a modulation/amplification unit 104.
The modulation/amplification unit 104 quadrature modulates the input analog signals and converts the base band signals into radio frequency (RF) band signals. The modulation/amplification unit 104 then amplifies the RF band signals for the output from an output terminal So. Upon this amplification in the modulation/amplification unit 104, the signals may suffer any distortions (amplitude distortions and phase distortions) by the amplifier.
An attenuation/demodulation unit 105 also receives the transmission signals in the form of feedback signals. The attenuation/demodulation unit 105 attenuates signals amplified by the modulation/amplification unit 104, into signals having a pre-amplified power. The attenuation/demodulation unit 105 then quadrature demodulates them and converts RF band signals into base band signals. The signals converted by the attenuation/demodulation unit 105 are fed to analog-to-digital converter (hereinafter referred to as ADCs) 106i and 106q, for the conversion into digital signals.
The digital signals are fed to a distortion compensation coefficient update unit 108 and to one input terminals of subtractors 107i and 107q. The other input terminals of the subtractors 107i and 107q receive digital signals xi and xq, respectively, from the input terminals Si and Sq, respectively. The subtractors 107i and 107q find difference (i.e., distortion upon the amplification in the modulation/amplification unit 104) signals between the digital signals xi, xq and output signals of the ADCs 106i, 106q, respectively, and feed the difference signals (error signals) to the distortion compensation coefficient update unit 108.
The distortion compensation coefficient update unit 108 finds a new distortion compensation coefficient hxe2x80x2 based on output signals of the ADCs 106i, 106q, output signals of the subtractors 107i, 107q and distortion compensation coefficient h from the distortion compensation coefficient storage unit 101. The unit 108 then updates the distortion compensation coefficient storage unit 101 by use of the new distortion compensation coefficient hxe2x80x2. This updated distortion compensation coefficient hxe2x80x2 is utilized for the subsequent input signal distortion compensations.
The above processings are iterated for each digital signal input.
In such a conventional transmitter, however, the feedback signals are converted by the ADCs 106i and 106q into digital signals. As described above, these feedback signals are obtained by subjecting the input signals to distortions upon the amplification and have the same level of amplitude value as the input signals. Due to the necessity to represent not merely the input signals but also the distortions, therefore, the ADCs 106i and 106q must have a high bit precision (i.e., a large bit number) and, because of the input signal handling, a high operation frequency. Accordingly as the input signals have a higher input bit rate, the conversion processing may possibly become too late. In the event that the input signals have an extremely large amplitude value and a high frequency as in the case of the CDMA system base station, this deficiency will become more prominent.
The present invention was conceived in view of such a situation. It is therefore an object of the present invention to provide a distortion compensating apparatus and a distortion compensating method which allow use of ADCs having a relatively low bit precision and having a relatively low operation frequency.
In order to achieve the above object there is provided a distortion compensating apparatus of the present invention for compensating distortions of an amplifier arranged to amplify analog signals converted from digital input signals, the distortion compensating apparatus converting the digital input signals into analog signals, the distortion compensating apparatus finding, in the analog region, distortion components of the amplifier from differences between the analog input signals and the analog output signals of the amplifier, the distortion compensating apparatus converting the distortion components into digital signals to thereby determine distortion compensation coefficients.
As a result, an analog-to-digital converter of the present invention handling only the distortion components for the conversion of analog signals into digital signals can be one having a lower bit precision and a relatively lower operation frequency than those of the conventional analog-to-digital converter for converting analog output signals from the amplifier into digital signals.
According to a first aspect of the present invention there is provided a distortion compensating apparatus for compensating distortions of an amplifier arranged to amplify analog signals converted from digital input signals, the distortion compensating apparatus comprising a distortion compensation coefficient application unit which holds distortion compensation coefficients for compensation of distortions of the amplifier, the distortion compensation coefficient application unit applying the held distortion compensation coefficients to the digital input signals; a first digital-to-analog converter which converts digital signals output from the distortion compensation coefficient application unit into analog signals, the first digital-to-analog converter providing the analog signals as its output to the amplifier; a first attenuator which attenuates analog output signals from the amplifier to reciprocal times the ideal gain of the amplifier; a second digital-to-analog converter which converts the digital input signals into analog signals; a subtractor which finds differential signals between output signals from the second digital-to-analog converter and output signals from the first attenuator; an analog-to-digital converter which converts the differential signals into digital signals; and a distortion compensation coefficient arithmetic unit which figures out new distortion compensation coefficients based on output signals from the analog-to-digital converter, the distortion compensation coefficient arithmetic unit using the new distortion compensation coefficients to update distortion compensation coefficients held by the distortion compensation coefficient application unit.
According to a second aspect of the present invention there is provided a distortion compensating apparatus for compensating distortions of an amplifier, comprising a first arithmetic circuit which effects in an analog region an arithmetic for obtaining distortion components of the amplifier from the differences between digital region signals associated with the input of the amplifier and analog region signals associated with the output of the amplifier; and a second arithmetic circuit which effects in a digital region an arithmetic of the distortion compensation coefficient for canceling distortion components, the distortion compensation coefficient being fed to input signals of the amplifier depending on the magnitudes of the distortion components.
According to a third aspect of the present invention there is provided a distortion compensating method for compensating distortions of an amplifier, the method comprising the steps of effecting in an analog region an arithmetic for obtaining distortion components of the amplifier from the differences between digital region signals associated with the input of the amplifier and analog region signals associated with the output of the amplifier; and effecting in a digital region an arithmetic of distortion compensation coefficients for canceling distortion components, the distortion compensation coefficient being fed to input signals of the amplifier depending on the magnitudes of the distortion components.
The distortion compensation coefficient application unit preferably includes a power calculation unit which calculates powers of the digital input signals; a distortion compensation coefficient storage unit which holds distortion compensation coefficients corresponding to power values of the digital input signals, the distortion compensation coefficient storage unit providing as its output distortion compensation coefficients corresponding to power values calculated by the power calculation unit; and a multiplier which multiplies the digital input signals by distortion compensation coefficients output from the distortion compensation coefficient storage unit.
The distortion compensating apparatus may further comprise a variable gain amplifier disposed between the subtractor and the analog-to-digital converter, the variable gain amplifier accepting the differential signals, the variable gain amplifier amplifying the differential signals by a variable gain to impart the amplified differential signals to the analog-to-digital converter; a second attenuator disposed between the analog-to-digital converter and the distortion compensation coefficient arithmetic unit, the second attenuator variably attenuating output signals of the analog-to-digital converter with an attenuation factor equal to the reciprocal of the gain of the variable gain amplifier; a control unit which provides a control of the gain of the variable gain amplifier and the attenuation factor of the second attenuator depending on the magnitude of the amplitudes or powers of the differential signals; a first switch which changes over signals fed to the analog-to-digital converter to either the differential signals or output signals of the variable gain amplifier; a second switch which effects a changeover such that the distortion compensation coefficient arithmetic unit receives either output signals of the analog-to-digital converter or output signals of the second attenuator; and a switch control unit which provides a control of the first switch and the second switch on the basis of absolute values of amplitudes of the differential signals, the switch control unit when the first switch is switched to the differential signals, changing over the second switch to the analog-to-digital converter, the switch control unit when the first switch is switched to the variable gain amplifier, changing over the second switch to the second attenuator.